The present disclosure relates to hydraulic fitting safety devices, particularly for use in airplanes or other aircraft.
In modern commercial airplanes, fuel is traditionally stored in the wings. A number of hydraulic lines may pass through the fuel storage area to provide power and control to hydraulically powered elements, such as wing flaps. These hydraulic lines require a number of fittings to connect various lengths of hydraulic tubing to each other, other fittings, and to bulkhead panels and direct the hydraulic fluid. During operation of the aircraft, it is possible that an electrical spark could be created between a hydraulic line and a fitting and propagate to the fuel tank, thereby causing a potential ignition source. This risk is contained or prevented in a number of ways, the present disclosure describes an alternative improved method and apparatus for inhibiting spark propagation from a fitting to the fuel tank.
The Federal Aviation Administration (FAA) has expressed concern regarding this potential ignition sources in the fuel tank of aircraft. Federal Aviation Regulation (FAR) 25.981(a)(3) (14 CFR 25.981(a)(3)) requires that any potential ignition source must be sufficiently contained by redundant ignition prevention measures. The specific language requires “that an ignition source could not result from each single failure, from each single failure in combination with each latent failure condition not shown to be extremely remote, and from all combinations of failures not shown to be extremely improbable.” This regulation generally requires that the system have triple redundancy, or three safety devices which would have to independently fail, in order to cause an ignition source to result in the aircraft fuel tank. Double redundancy may be sufficient for permanent installations where the ignition prevention measures are shown to be highly reliable.
Current methods of satisfying this requirement focus on reducing the possibility of sparks between the hydraulic line and fitting. One potential source of sparks is electrical current flowing along the hydraulic lines. The electrical current may jump from a hydraulic line to a fitting, thereby causing a spark. One method in use is dissipating or directing electrical current away from the hydraulic lines so that electrical current does not pass through the fuel tank. For example, in-line static dissipaters may be used to prevent electrical current. These methods of mitigating this risk may require a large number of parts, increasing cost, complexity, and installation time.
Some existing fitting designs include a polymeric liner within the fittings to protect tubes and fittings from wear in surface. Without the use of electrical dissipation this liner could exacerbate sparking problems when dielectric breakdown occurs near the tip of the fitting if the fitting is forced to carry large amounts of electrical current due to a lightning strike. Hydraulic tubes in carbon fiber reinforced plastic (CFRP) wings will potentially carry higher current levels than similar fittings in metal wing airplanes. Additional safety features may therefore be required to ensure safety and compliance with Federal regulations.
In-line electrical isolators that disrupt current flow may be used to prevent current flow through the hydraulic line, thereby preventing sparking due to electrical current. These in-line static dissipaters are generally electrically non-conductive tubes that are inserted in the hydraulic line. These non-conductive tubes resist current flow, thereby causing current to flow through structure other than the hydraulic line. This system may not be desirable because it adds additional weight to the system and prevents the use of hydraulic lines as a means to conduct electrical current.
Another source of sparking may be hot material that is forced out from the fitting under pressure. This hot material may be lubricant used to protect the fittings, molten metal from internal sparking, or any other material which may become heated due to pressure or resistance to electrical current and forced out from the hydraulic fitting.
With the increased desire for light weight composite or otherwise non-conductive (or low-conductivity) materials for the fuel tank and other aircraft structure, it may not be desirable or possible to transfer electrical current from the hydraulic lines to the fuel tank to eliminate the risk of sparking. Additionally, a composite or non-conductive fuel tank may build up precipitation static as the aircraft travels through the air. This precipitation static must be dissipated away from the fuel tank to prevent sparking or damage to the fuel tank. Conductive hydraulic lines may be utilized to transfer electrical current or dissipate precipitation static from the fuel tank.
Therefore, there is a recognized need in the art for a method and apparatus for increasing safety of hydraulic joints by preventing sparks from propagating from a hydraulic joint to the surrounding medium while maintaining conductive properties of the hydraulic line.
There is further recognized a need in the art for safety materials which are useful in dissipating precipitation static from a composite fuel tank surface while preventing spark propagation from the hydraulic line to the surrounding medium.